Our stomach has an enzyme called pepsin, which digests
proteins into polypeptides and our small intestine also has an enzyme
called trypsin which digests protein into
polypeptides too. Since our stomach has an enzyme which can digest protein,
why does protein digestion still take place in small intestine?
The enzymes you a referring to are generally called
'proteases', proteins that degrade other proteins.
They do not do this randomly, but use a specific
amino acid sequence to cleave. And that iwhy we
produce different proteases in our digestive tract: to
chop up as much as protein, in as small fragments as
possible. After all, not every protein contains
sufficient target sequences for trypsin or pepsin to
generate small enough fragments.
Nelly - Pepsin only begins the digestion of proteins as it breaks specific
bonds, adjacent to precise amino acids; it leaves many other bonds intact.
The enzymes in the small intestine finish the job, breaking the rest of the
bonds that link amino acids together to make proteins.
Is that what you need?
Because proteins are very long complicated molecules. Proteins have 3 levels
of complexity. The most complex level is the 3 dimensional level. Pepsin
helps to "unwind" the proteins and breaks the bonds between the amino acids
in certain places. The food passes to the small intestine and other enzymes
break the bonds between different amino acids than pepsin. Because proteins
are such complicated molecules it takes a long time and more than one enzyme
to completely break them down into amino acids.
All polypeptides are composed of amino acids which have various side chains
that distinguish the 20 different amino acids from each other. Some of these
side chains carry a positive charge; some, a negative charge; and some, no
charge at all. To break down a peptide to usable components the digestive
system must be designed to attack all the different charges. In the acidic
environment of the stomach, the negative side chains are addressed by pepsin.
In the more alkaline environment of the intestine, the positive side chains
are digested by trypsin. Each of these enzymes are similar to each other but
through evolution have been selected for stomach and intestine because pepsin
works better in an acidic environment and trypsin works better in alkaline
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Update: June 2012